Methods to make polyethylene compositions comprising two (or more) polymer components, for example high and low molecular weight components, are well known in the art. These types of polymers, known generally as bimodal or multimodal polymers, can be useful for a huge range of applications which span from low density film, to high density pressure pipe.
One method to make such compositions involves taking two different ethylene polymers, for example polymers which differ in molecular weight and/or comonomer content, and blending them in a post-reactor extrusion or melt blending process. Another well known process involves using a polymerization catalyst in two sequentially arranged polymerization zones, where each zone provides distinct conditions, such as high and low concentrations of hydrogen, to form in situ, a blend of low and high molecular weight polymers respectively.
Multi-component blends can also be made in a single reactor by using at least two polymerization catalysts which provide divergent polymers under the same set of reactor conditions. Such multi component catalysts have taken many forms over the years and most typically involve mixed Ziegler-Natta catalysts, mixed Ziegler-Natta and single site catalysts (such as metallocene catalysts) or mixed single site catalysts.
Mixed catalysts consisting of a chromium catalyst and a so called “single site catalyst” have also been explored, but to a lesser extent. For example, E.P. Pat. No. 339, 571 discloses catalyst systems for use in the gas phase and which involve the combination of a chrome oxide catalyst and a metallocene catalyst. The catalyst components were supported on a silica support. Similarly, in U.S. Pat. No. 6,541,581, a chrome oxide catalyst is co-supported with a zirconocene catalyst on an inorganic oxide support.
In U.S. Pat. No. 5,723,399 a chromium catalyst, such as a silyl chromate catalyst, is combined with a metallocene or a constrained geometry catalyst in a single reactor. The catalyst components were co-supported on a silica support or alternatively, a metallocene or constrained geometry catalyst was added to a supported chromium catalyst in situ.
Catalysts comprising a silyl chromate catalyst and a group 4 single site catalyst which has at least one phosphinimide or ketimide ligand have been disclosed in commonly assigned Canadian Pat. Appl. Nos 2,605,044 and 2,605,077.
For multi component catalysts the use of process control knobs such as hydrogen concentration to control melt index and other resin specifications can be a challenge and can lead to undesirable polymer compositions, since each catalyst component will typically have a different response to the parameter being changed. For example, a bimodal or multimodal polymer may become unimodal at different hydrogen concentrations due to the different hydrogen response of each catalyst component present. Mitigation of unintended fluctuations in polymerization conditions, such as temperature excursions or impurity levels is also a challenge with multi component catalysts, as each parameter change may have a differential impact on the performance of each catalyst species present. For systems in which distinct catalysts are fed separately to a polymerization zone, it is sometimes possible to control polymer characteristics (e.g. melt index, polydispersity, comonomer distribution, etc.) by changing the relative amounts of each catalyst present in the polymerization zone. However, multi component catalysts are often co-supported, especially for use in gas phase or slurry phase polymerization in order to make well mixed or homogeneous polymer compositions. For co-supported catalyst systems, the amount of polymer produced by each catalyst species is generally fixed by the initial ratio of catalyst components present on a support. It is therefore desirable to have methods which can attenuate product drift or to control polymer compositions made with a multi component catalyst, without having to reformulate the catalyst.
In-situ methods which alter polymer compositions made by a co-supported multi catalyst formulation have been explored. One manner in which the polymer compositions have been controlled has been to use a so called “make up catalyst”. In U.S. Pat. No. 6,410,474, this involves the addition of a separate catalyst which is of same type as one catalyst species present in a multi catalyst system. This allows one to increase the amount of polymer made by one or the other of the catalyst species of the multi catalyst system. In this way, the ratio of polymer components can be altered in situ. The separate feeding of two multi component catalysts, each having a different ratio of catalyst species has also been used to control the polymer composition in situ, as is disclosed in U.S. Pat. Nos. 6,462,149 and 6,610,799. These methods suffer from the need for an additional catalyst delivery stream and can produce polymers having poor homogeneity, since the separate addition of a make-up catalyst will initiate growth of a separate polymer particle.
Another in-line method to control co-supported multi component catalysts, is to change the relative activities of each active species by the introduction of a catalyst poison. As described in U.S. Pat. No. 5,525,678, catalysts composed of a Ziegler-Natta and metallocene species can be controlled through the introduction of carbon dioxide or water.
Similarly, U.S. Pat. No. 6,828,395 teaches the use of “control agents” such alcohols, ethers, amines, or oxygen to alter the properties of a bimodal polymer made by a “bimetallic catalyst”. To make the bimetallic catalyst, a Ziegler-Natta catalyst was co-supported with a metallocene catalyst.
In U.S. Pat. No. 6,995,219, a series of “adjuvants” were explored, for their ability to modify the relative activities of bridged and unbridged metallocenes which were used in a “multi-site” catalyst formulation. The adjuvants which were selected from the group consisting of phosphines, phosphites, acetylenes, dienes and acetyls, preferentially decreased the activity of the bridged metallocene, which had the effect of lowering the ratio of high to low molecular weight components produced during polymerization. In addition to modifying activity, the adjuvants also changed the molecular weight performance of each catalyst species. This is not always desirable, and it would be useful if the relative amounts, and the relative molecular weights of different polymer components could be controlled independently.
Canadian Pat. Appl. No. 2,616,053AA demonstrates the effect of adding water or carbon dioxide to a “hybrid” catalyst comprising a late transition metal catalyst and a metallocene catalyst. Water had the effect of reducing the relative activity of the late transition metal catalyst which made a low molecular weight component, while carbon dioxide reduced the relative activity of the metallocene catalyst responsible for making a high molecular weight component. In this way, water and carbon dioxide were used to increase and decrease the high to low molecular weight ratio respectively, of polymer components made in a single reactor. Although further gas phase catalyst types are broadly disclosed, including chromium catalysts, there is no teaching of the relative effects of carbon dioxide on chromium catalysts and group 4 based single site catalysts.
In light of the above, there remains a need for methods to control the performance of other mixed or multi component catalyst systems, especially systems which comprise a chromium catalyst in combination with a group 4 single site catalyst.